With Firefox 1.0, I used “single window mode” to force all new windows to instead open in new tabs. With Firefox 1.5, most new windows are redirected to tabs but any window with a custom size will still get its own window. I (and many others) prefer that all new windows be turned into tabs. To do this, go to about:config and set the browser.link.open_newwindow.restriction variable to 0. This was changed as a result of bug 313300; they felt that 2 (resized windows don't become tabs) was a better default.